Flextensional transducers such as those illustrated in U.S. Pat. Nos. 3,274,537 issued Sept. 20, 1966, and U.S. Pat. No. 3,277,433 issued Oct. 4, 1966, to W. J. Toulis in general are characterized by a flexible outer shell and a piezoelectric stack of elements used in a length expander mode which is placed between opposing interior walls of the shell. When actuated, the stack expands and contracts, thereby flexing the shell which, in turn, is coupled to an acoustic medium so as to project acoustic energy into the water.
While these types of transducers are exceptionally efficient, the performance of the transducers varies with depth and is limited in maximum depth by the amount of prestress that can be imposed on the piezoelectric stack to avoid exposure to tensile stress.
As is well known, piezoelectric properties of ceramic transducers vary with stress, with the stress varying as a function of the depth of the transducer in water, since increased hydrostatic pressures cause increased shell deflection. Thus, the characteristics of the transducer are variable with depth and, in general, the maximum depth of operation of the piezoelectrically driven flextensional transducer is governed by allowable ceramic stress and performance degradation. In part, hydrostatic pressure may be compensated for by filling the shell of the flextensional transducer with liquid. However, liquid filling requires complex decoupling devices, and this is generally undesirable due to the effect of the liquid fill on the transducer characteristics.
The problem of driving a flextensional transducer at increased ocean depths is solved in subject invention by the utilization of a magnetically driven element, which, in one embodiment, employs a moving coil in a magnetic field. This device is used in place of the piezoelectric stack and is, in general, located between opposing interior walls of the flextensional transducer's shell. In one embodiment, a permanent magnet and pole pieces are mounted to one interior wall, with the moving coil mounted to a diametrically opposite interior wall. The shell is driven by energizing the coil which causes the coil to move toward or away from the pole pieces thereby flexing the walls of the transducer inwardly or outwardly. For elliptical shells, the magnetically driven element may lie either along the major or minor axis. With electrodynamic drive, the minor axis is preferred because the coil is a low impedance drive and the shell in this direction also has a low impedance, offering a good match for maximum power transfer to the medium. Location along the minor axis also facilitates alignment and ease of fabrication because of the shorter distance between the interior walls.
The advantage of utilizing such a magnetically driven element is that there is no variation of performance with depth because the driving element is not subjected to depth dependent stresses. This is because the drive coil is free to move with respect to the pole pieces which surround it in response to the flexure of the walls of the transducer due to hydrostatic pressure increases with increasing depth. To insure linear drive characteristics the coil length is extended beyond the gap sufficiently to accommodate shell deflection at maximum depth.
In an alternative embodiment, the magnetically actuated device may include a magnetostrictive rod placed between opposing interior walls of the shell in which the magnetostrictive rod is overwound with an electrical coil. When energized, this coil causes the magnetostrictive rod to expand and contract in a longitudinal direction thereby causing flexure of the shell. It should be noted that for magnetostrictive rods, pressures at the ends of the rod do not cause the same distortion in molecular alignment as created in a ceramic material, such that transducer parameters are not affected by the increased hydrostatic pressures at increasing ocean depths. In addition, since metals perform equally well in tension as compression the need for prestress of the stack has been removed, extending depth capability of the shell.
It is therefore an object of this invention to provide an improved flextensional transducer;
It is still further object of this invention to provide a magnetic drive for a flextensional transducer;
It is another object of this invention to provide a drive for a flextensional transducer which is depth independent and in which the driving element is either not subjected to stress due to depth or is relatively insensitive to depth related stress;
It is another object of this invention to provide a depth independent response characteristic for a flextensional transducer.
These and other objects of the invention will be better understood in connection with the appended drawings and the following detailed description wherein